Insulating Joints

With underground installations in the soil, it must be ensured that no water can penetrate in gaps between cathodically protected and unprotected parts since the cathodically unprotected side of the coupling can be destroyed by anodic corrosion. Sections of pipe behind the insulator must be particularly well coated. 

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Fig. 9-17 Interference of a long (by shorting insulating joints) and a short pipeline by an anode voltage cone 1 km in length P - potential measuring point / = insulating...

Fig. 10-4 Insulating Joint with explosion-proof spark gap (I/sohz = 1 1/50//s = 2.2 kV surge current =100 kA). 1, insulating flange 2, hexagonal nut 3, insulator ring 4, insulator sleeve 5, insulator disc 6, steel disc 7, blue asbestos gasket 8, explosion-proof spark gap.

Fig. 10-8 Pipe/soil potentials and protection currents for a pipeline. Drainage test x-x after 1 year o-o. P = potential test point R = pipe current test point LA = cathodic protection station / = insulating joint SP = pipe casing potential test point.

Fig. 10-9 Internal cathodic protection to avoid the danger of anodic corrosion behind an insulating joint in a brine pipeline.

If the protection current becomes too high due to this connection in cathodi-cally protected tank installations, then insulating joints are usually installed in the pipeline from the filling nozzle. Care must be taken that the continuity bond is not broken. If there is a danger of stray currents with dc railways due to a permanent connection between track and filling equipment, the continuity bond should be applied only during the filling process. 

On stretches of electric line, according to Ref. 12, filling tracks without overhead conductors are usually isolated from the rest of the rail network with insulating joints in order to keep the rail currents flowing to the tank installation as... 

In modem PMR construction, thermal insulation that is unaffected by water or that can be kept dry in some manner is required. Extruded polystyrene (XEPS) foam insulation boards are commonly employed (see Insulation, THERMAL). They are placed on top of the waterproofing roof membrane, which is next to the deck. The insulation should not be adhered to the membrane. Ballast at the rate of >48.8 kg/m2 (1000 lb/100 ft2) holds the insulation in place and offers protection from the sun. The insulation joints are open and drainage must be provided. Various other materials, eg, patio blocks and concrete slabs, are also used as surfacings and ballast. The extra weight imposes more exacting requirements on construction. 

A — Water seal, I) — Diaphragm, F — Insulating joint, K — Cathode of perforated MS sheot, fV — Supporting frame. 

Stray current flowing along a pipeline very often will not cause damage inside the pipe, because of the high conductivity of the electric path compared with the electrolytic path. The damage occurs when the current reenters the electrolyte and will be localized on the outside surface of the metal. If the pipe has insulated joints and the stray current enters the internal fluid, localized corrosion on the internal side of the pipe will occur. The best solution to avoid this mode of corrosion is the electrical... 

FIGURE 12.29 Situations of coupling of different materials with the formation of galvanic couples, (a) Possible sites for corrosion unfavorable, (b) Galvanic coupling is avoided or its effect is somewhat reduced by the interposition of insulating joints or intermediate couplings preferable. 

If galvanic couplings are avoided by using insulating joints, we must pay attention to the fact that the framework structures do not realize the electrical connection that the use of insulating joints was meant to avoid (Figure 12.40). 

If insulating joints are installed in the above-mentioned pipe in order to reduce stray-current pickup, corrosion is now focused on the water side of the joint where any current that persists leaves the pipe to enter the water. Or, if a high-resistance joint exists between two sections of a buried pipe, corrosion may be more pronounced on the side where current enters the soil (Fig. 12.3). 

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